Crosslinkers used for aliphatically unsaturated organopolysiloxanes are almost exclusively organopolysiloxanes containing hydrogenmethylsiloxane units, in the simplest case hydrogenmethylpolysiloxane end-blocked with triorganosiloxy groups. To increase the reactivity, it is useful to incorporate dimethylsiloxane units between the hydrogenmethylsiloxane units by equilibration. However, these measures are only limited improvements.
Apart from these crosslinkers based on hydrogenmethylsiloxane units, crosslinkers without methyl groups are also known. DE-B 1 955 511 and the corresponding U.S. Pat. No. 3,615,272 describe resins comprising hydrogensiloxane units, i.e., trifunctional units. However, such polymers are for restricted utility use as crosslinkers in addition-curing organopolysiloxane compositions since they hardly dissolve in the crosslinkable diorganopolysiloxanes.
EP-A 568 318 discloses organopolysiloxanes containing trifunctional siloxane units and monofunctional siloxane units containing Si-bonded hydrogen. According to EP-A 568 318, T units in the form of organosiloxane units are end-blocked with hydrogendimethylsiloxy groups, with a differing number of dimethylsiloxane or hydrogenmethylsiloxane units being incorporated between these groups.
According to DE-A 37 16 372, organopolysiloxanes containing Si-bonded hydrogen atoms and from 3 to 5 silicon atoms per molecule are prepared from [H(CH.sub.3).sub.2 Si].sub.2 O and trialkoxysilanes in a hydrolytic process.
Polym. Prepr. Vol. 34(2), 538 (1993) discloses that the self-cyclization of allyltris(dimethylsiloxy)silane is dominant when this compound reacts in dilute form under platinum catalysis.
Polym. Prepr. Vol. 34(2), 503 (1993) describes the self-cyclization. To avoid this, attempts are made to incorporate relatively long alkenyl groups into the monomeric compound of the formula alkenylSi(R.sub.2 SiH).sub.3. Isomerization of the .omega.-alkenyl double bond into an internal position takes place to an extent of from 15 to 16 mol % in the synthesis of the monomeric compound, as a result of which this percentage remains unreactive. In addition, the Si--H groups are further "diluted" because of the higher weight of C.sub.6 H.sub.11 or C.sub.8 H.sub.15 groups. The end products are unstable since the Si--H groups have to be protected to avoid gel formation.
According to Chemical Abstracts, Vol. 122, 56637, 1995, vinyltris(dimethylsiloxy)silane is prepared in a complicated process in which the volatile dimethyl-chlorosilane and the highly inflammable diethyl ether are required. In addition, auxiliary bases are needed for the work-up. The polymerization under various conditions forms self-polymers having average molecular weights of from 5,200 to 53,000 g/mol. Although hardly any cyclic products are formed, the polymerization can be controlled only with difficulty since the silane reacts strongly exothermically as soon as the platinum catalyst is added. The use as crosslinker is not described.
U.S. Pat. No. 5,097,054 describes a process for preparing compounds of the formula R.sub.4-n Si(OSiMe.sub.2 H).sub.n, with only aliphatically saturated radicals and no aliphatically unsaturated radicals being described as radicals R.